Matthew & John,
My normal reaction when someone says a risk is "so small as to be
practically nonexistent", is to figure that:

They are overconfident about the reliability of their estimates of
how small the risk actually is; OR

They have an idea of "practically nonexistent" that doesn't fit with
the way the rest of society uses that term.

Strangely neither of those explanations allows me to live in peace, free
from the fear of accidents.

Actually, one in a million per crash probably does qualify as "practically
nonexistent". The problem is that that figure isn't the risk of a premature
nuclear detonation, it is the risk of a detonation from "random"
reliability-based events.
Strangely, those qualifications (with the implication that the full risk of
the event including non-random events may be multiple orders of magnitude
more likely) never make it into the confident declarations of safety.

> John,>> The current US requirement for nuclear weapons safety during a crash is a> probabilty of one in a million of a premature nuclear detonation. I guess> that doesn't really qualify as 'practically nonexistent'.>> That being said, the nuclear weapons safety community has spent an awful> lot of time and money thinking about safety in the wake of such accidents> as Goldsboro, see their 3I principles for example, and I believe there> are broader architectural lessons that can be learned and transferred to> other domains.>> See the references in my post for further details.>>> http://criticaluncertainties.com/2010/03/21/lessons-from-nuclear-weapons-safety/>> Regards,>> On Wednesday, 2 October 2013, John Downer wrote:>>> Further to earlier discussions on the safety of the bomb (and courtesy of>> my former colleague Anne Harrington):>>>> From the Guardian: "US nearly detonated atomic bomb over North Carolina –>> secret document">>>> "A secret document, published in declassified form for the first time by>> the Guardian today, reveals that the US Air Force came dramatically close>> to detonating an atom bomb over North Carolina that would have been 260>> times more powerful than the device that devastated Hiroshima.>>>> The document, obtained by the investigative journalist Eric Schlosser>> under the Freedom of Information Act, gives the first conclusive evidence>> that the US was narrowly spared a disaster of monumental proportions when>> two Mark 39 hydrogen bombs were accidentally dropped over Goldsboro, North>> Carolina on 23 January 1961. The bombs fell to earth after a B-52 bomber>> broke up in mid-air, and one of the devices behaved precisely as a nuclear>> weapon was designed to behave in warfare: its parachute opened, its trigger>> mechanisms engaged, and only one low-voltage switch prevented untold>> carnage.">>>>>> http://www.theguardian.com/world/2013/sep/20/usaf-atomic-bomb-north-carolina-1961>>>>>> For context, here's the official government assessment from 1960: "Stay>> Safe, Stay Strong: The Facts about Nuclear Weapons">> http://archive.org/details/StaySafe1960>>>> My favorite bit is at minute 20:00:>>>> So how safe is a nuclear bomber coming in for a crash landing?>> "...the possibility of an accidental nuclear explosion is so small as to>> be practically nonexistent...you and your family may live in peace, free>> from the fear of nuclear accidents">>>>>>>>>> --------->> Dr. John Downer>> SPAIS; University of Bristol.>>>>>>>>>>>>>>>> --> *Matthew Squair*> MIEAust CPEng>> Mob: +61 488770655> Email: MattSquair_at_xxxxxx
> Website: www.criticaluncertainties.com <http://criticaluncertainties.com/>>>>> _______________________________________________> The System Safety Mailing List> systemsafety_at_xxxxxx
>>